1. Field of the Invention
The present invention relates to a decoding technique for encoded image data.
2. Description of the Related Art
An encoding technique for image compression for a reduction in file size is indispensable for efficiently storing, managing, and transferring digital images. In general, when image data is to be encoded, the image is divided into several blocks, and encoding processing is performed on a block basis. When encoding is performed on a block basis, block boundaries tend to distort. Under the circumstance, an encoding technique that suppresses distortion at block boundaries is known (for example, Japanese Patent Laid-Open No. 2006-197573 to be referred to as reference 1 hereinafter). The technique disclosed in reference 1 divides an image into tiles. The technique then generates hierarchical stream data by repeating DCT for each tile. In order to suppress block distortion that occurs between tiles, this technique performs predetermined processing by using the pixels of two adjacent tiles before the execution of DCT or in the process of executing DCT. As such encoding processing, JPEG XR is known (e.g., “Current report of standardization for the JPEG XR image compression”, The Journal of The Institute of Image Electronics Engineers of Japan, Vol. 37, No. 4, pp. 502-512, issued Jul. 25, 2008, and Nikkei Electronics, pp. 71-77, issued Dec. 29, 2008).
Processes based on the above technique include the processing of suppressing block distortion in the process of performing stream conversion processing. The user can select a process of his choice from these processes. Although this function can suppress distortion, the calculation cost for stream conversion processing increases double or more. The function of suppressing distortion includes processes in encoding processing and decoding processing that is designed to be performed in pair. That is, when a process has been performed at the time of encoding, a corresponding process is automatically performed at the time of decoding. A decoding apparatus, which decodes data encoded upon distortion suppression, always needs to perform a process corresponding to distortion processing, resulting in a delay in display, no matter how the user wants to display the data at high speed.
The maximum resolutions of recent digital cameras have been increasing. Such cameras generally have resolutions exceeding 10,000,000. However, display devices used in personal computers and the like have much lower resolutions than digital cameras. For the purpose of browsing instead of image editing and the like, therefore, it is sufficient for such display devices to display images having resolutions lower than original resolutions, and the devices are required to have high response between the instant an image to be displayed is designated and the instant the image is displayed.